In a typical thermal bubble inkjet printing system, an inkjet printhead ejects ink droplets through a plurality of nozzles toward a print medium, such as a sheet of paper, to print an image onto the print medium. The nozzles are typically arranged in one or more arrays, such that properly sequenced ejection of ink from the nozzles causes characters or other images to be printed on the print medium as the printhead and the print medium are moved relative to each other.
Thermal inkjet printheads eject droplets of fluid from a nozzle by passing electrical current through a heating element to generate heat and vaporize a small portion of the fluid within a firing chamber. The current is supplied as a pulse which lasts on the order of 2 micro-seconds. When a current pulse is supplied, the heat generated by the heating element creates a rapidly expanding vapor bubble that forces a small droplet out of the firing chamber nozzle. When the heating element cools, the vapor bubble quickly collapses. The collapsing vapor bubble draws more fluid from a reservoir into the firing chamber in preparation for ejecting another drop from the nozzle. Unfortunately, because the ejection process is repeated thousands of times per second during printing, the collapsing vapor bubbles can also have the adverse effect of damaging the heating element. Collapse of the vapor bubble leads to cavitation damage to the heater surface material. Each of the millions of collapse events ablates the coating material. Once ink can penetrate the layer or surface material on the heating element and contact the hot, high voltage resistor surface, rapid corrosion and physical destruction of the resistor soon follows.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.